Zinc oxide particles, which have been used as a sunscreen ultraviolet blocking agent in cosmetic product applications, are ultrafine particles having an average particle diameter of 0.1 μm or less, and have a particle shape that is not controlled to be a hexagonal plate-shape. However, such conventional zinc oxide ultrafine particles for ultraviolet blocking deteriorate slippage, and are therefore hardly used for cosmetics in which comfort in use is important, such as foundations and other make-up cosmetics. Such make-up cosmetics generally contain plate-shaped particles such as those of talc, mica and barium sulfate.
However, these plate-shaped particles do not have an ultraviolet blocking effect, and therefore for imparting ultraviolet blocking performance, zinc oxide fine particles or titanium oxide fine particles in an amount small enough that slippage is not hindered, or an organic ultraviolet blocking agent must be used in combination. Accordingly, presence of plate-shaped zinc oxide particles having proper slippage is preferable because slippage and ultraviolet blocking performance can be imparted with one kind of particles.
In recent years, cosmetics having an effect of blurring a base when applied to the skin (a so called soft focus effect) have been known. However, no attempt has been made to obtain such a soft focus effect with zinc oxide particles.
In sunscreen cosmetic applications, zinc oxide fine particles having a particle diameter of 0.1 μm or less have been used. However, such zinc oxide fine particles have the disadvantage that they are poor in soft focus effect, and lack an effect of blurring a base.
As hexagonal plate-shaped zinc oxide, those in Patent Documents 1 to 3 are known. However, the zinc oxide particles in Patent Document 1 are formed by aggregation of zinc oxide fine particles in a hexagonal plate shape, and therefore proper slippage cannot be achieved. Further, since the particle shape is not sufficiently controlled, physical properties tend to vary, so that it is difficult to obtain a cosmetic with stable quality.
Patent Document 2 describes flaky plate-shaped zinc oxide particles, and uses thereof in cosmetics and industrial applications. However, many of the flaky plate-shaped zinc oxide particles described in Patent Document 2 have a large particle diameter, and the particle shape is not controlled to be fine and uniform. In the production method, a basic zinc salt is generated, and therefore a thermal decomposition step such as that of calcinating is required to obtain zinc oxide.
Patent Document 3 describes hexagonal plate-shaped zinc oxide particles. However, the zinc oxide particles in Patent Document 3 significantly vary in particle diameter and shape, and are aggregated, so that problems such as those described above cannot be sufficiently rectified.
In applications of heat releasing materials in electronic/electrical fields, aluminum oxide, aluminum nitride, boron nitride, zinc oxide and the like are often used as a heat releasing filler. It is generally known that two or more kinds of spherical particles having different particle diameters are filled, and a filler is filled in a resin or the like at a high rate for achieving high heat releasing performance. Meanwhile, it is considered to make use of anisotropy of thermal conduction by compounding particles of different shapes, such as plate-shaped particles and needle-shaped particles, for more effectively enhancing heat conductivity. However, the zinc oxide particles in Patent Document 1 are aggregates, and therefore influences of heat resistance between particles in plate-shaped particles are significant, so that anisotropy of thermal conduction specific to plate-shaped particles cannot be exhibited.
Usually, in production of zinc oxide particles, mostly a calcinating step is essential. In calcinating, however, equipment capable of performing a treatment at a high temperature is required, and there is the problem that the particle diameter and the particle shape which have been adjusted/controlled in the stage of a zinc oxide precursor are changed by calcinating, and so on. Therefore, it is very preferable to obtain zinc oxide particles without performing calcinating.